When uploading textures from an unpack buffer, some drivers expect an
extra row paading, causing them to think the pixel buffer is not big enough.
We work around this by uploading the last row separately.

BUG=angleproject:1512

Change-Id: I52fb8b35dc450b957f1fafb0b405c81bf0504157
Reviewed-on: https://chromium-review.googlesource.com/385193
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>